|What firewall are you using?|
If you try to forward ports in the routers firewall, you need to go to the firewall and NAT settings and create a new rule to forward a port to a specific ip address and port.
Assuming you want to forward incoming TCP port 5901 to the ip address 192.168.0.20 port 5901.
source port protocol destination ip address destination port
5901 TCP 192.168.0.20 5901
You should find detailed informations in the routers manual on howto do this on your router.
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